1. Field of the Invention
The present invention relates to a multi-layered mirror, a manufacturing method thereof, a stress control method thereof, and an exposure apparatus. More particularly, the present invention relates to a multi-layered mirror for X-rays or neutrons, such as X-ray apparatus including X-ray telescopes, X-ray lasers, and soft X-ray projection lithography.
2. Description of the Related Art
The complex index of refraction of a substance is expressed by the following equation: EQU n=1-.delta.-i.multidot.k (1)
The .delta. and k parameters are each quite small relative to unity in the X-ray region, so reflective optical systems are used in the X-ray region. Within Equation (1), i.sup.2 =-1, and k of the imaginary term indicates the absorption of X-rays by the substance.
Lenses utilizing refraction can not be used in the X-ray region since the difference between the refractive index of substances in the X-ray region and the refractive index of a vacuum (=1) is extremely small. Furthermore, even when reflection is used, reflectivity is very low.
Since the refractive index is slightly below 1, a high reflectivity is displayed when X-rays strike a smooth surface at a small grazing incidence tilt angle since total reflection occurs below an angle (critical angle of total reflection) determined by the refractive index. This type of mirror is called a "grazing incidence mirror."
However, the X-ray reflection angle of a grazing incidence mirror is greatly restricted. Therefore, such a grazing incidence mirror is deficient because it is impossible to construct an optical system with the multiple elements required for correction of aberration.
Accordingly, by using multiple reflecting surfaces and by keeping the X-rays reflected from each reflecting surface in phase, a multi-layered mirror with a high reflectivity was developed. This multi-layered mirror uses a film using a first substance with a small refractive index (a first layer) and a film using a second substance with a large refractive index (a second layer). The two types of films are alternately applied upon a substrate
Whether the first substance layer or the second substance layer is adjacent to the substrate is not important. For example, a multi-layered mirror with molybdenum(Mo)/Silicon(Si) alternately multi-layered films has a reflectivity of 60% or greater for soft-X rays with 13 nm in wavelength and perpendicular incidence.
Because reflectivity is extremely small for nearly perpendicular incident angles that are even smaller than the totally reflecting limit angle .theta.c of a grazing incidence optical system, a reflective optical system used in the X-ray region uses a multi-layered film. Specifically, this type of multi-layered mirror is utilized in various fields of X-ray optics for applications such as X-ray telescopes, X-ray microscopes, X-ray reducing projection exposure devices, X-ray laser resonators, etc.
A multi-layered mirror used in such a reflective optical system is formed as laminated layers of two types of substances with high amplitude reflectivity at the interface. The thickness of each layer is determined based upon optical interference theory and is selected so that the waves reflected from each interface are in phase. Among the layered substances of such a multi-layered mirror, one substance has a small refractive index difference from vacuum (refractive index=1), and the other utilized substance has a large refractive index difference.
Since a multi-layered mirror can reflect X-rays perpendicularly, an optical system utilizing perpendicular reflection can have lower aberrations than a grazing incidence optical system utilizing total reflection. Furthermore, as indicated by Equation (2), a multi-layered mirror features wavelength selectivity since X-rays are strongly reflected only when the Bragg condition given below is satisfied: EQU 2d.multidot.sin .theta.=m.multidot..lambda., (2)
where d is the multi-layered film periodic length, .theta. is the incident tilt angle, .zeta. is the X-ray wavelength, and m is the order.
Previously known examples of a multi-layered film used for a multi-layered mirror include a W/C multi-layered film prepared by alternately laminating layers of W (tungsten) and C (carbon) and Mo/C multi-layered film prepared by alternately laminating layers of Mo (molybdenum) and C. Such multi-layered films are formed by thin film growth technology such as sputtering, vacuum deposition, CVD (Chemical Vapor Deposition), etc.
Among the multi-layered films used for such multi-layered mirrors, the long wavelength side of the L absorption edge (12.6 nm wavelength) of Si of a Mo/Si multi-layered film has high reflectivity, and a comparatively good multi-layered film can be produced that has 60% or greater reflectivity (perpendicular incidence) in the vicinity of 13 nm wavelength. Mirrors utilizing this Mo/Si multi-layered film are used for research in X-ray telescopes, X-ray microscopes, X-ray reducing projection exposure devices, X-ray laser resonators, etc. It is anticipated that such mirrors will be used for reduction copying lithographic technology utilizing soft X-rays, which is called "EUVL" (Extreme Ultraviolet Lithography).
While the sputtering method has produced high reflectivity Mo/Si multi-layered mirrors, thin layers formed by the sputtering method are known to generally have internal compressive stress (Sey-Shing Sun, Internal Stress in Ion Beam Sputtered Molybdenum Films, J. Vac. Sci. Technol. A4 (3), May/June, 1986). When such internal stress occurs within a Mo/Si multi-layered film, the internal stress causes deformation of a substrate of a multi-layered mirror. This deformation causes wave front aberrations in the optical system, resulting in degradation of optical characteristics.
Since the refractive indexes of substances are also extremely near unity for a neutron beam, multi-layered mirrors are used for neutron beams in the same manner as X-rays. Although perpendicular reflection is impossible due to the short wavelength (high energy) of a neutron beam, the critical angle of total reflection can be increased by formation of a multi-layered film upon a surface of a grazing incidence mirror.
Furthermore, in addition to X-ray and neutron applications, a multi-layered mirror of the present invention can be used for ultraviolet light, visible light, and infrared light.